Various suspension devices using, for example, a coaxial spring and hydraulic shock absorber are known in this field. Such devices are, for example, described in patent FR 2 897 915 (“système d'amortissement à détente rapide monté en jambe de force [rapid-relaxation shock absorber system mounted as a strut]”) by the same applicant. In the known way, a shock absorber comprises a body of tubular shape the ends of which are closed by walls, and in which a piston extended by a rod that passes through one of the end walls can move in a translational movement, the said piston delimiting two chambers. In the case of a hydraulic shock absorber, bores between the two chambers are commonly provided so as to allow the fluid to pass from one to the other according to predetermined laws. In addition, means are potentially provided to compensate for the variation in volume of the rod.
Such shock absorbers are usually associated with suspension systems, of the coil spring type, which give the suspension an elastic movement function.
Document U.S. Pat. No. 8,123,006 describes a suspension fork including a pneumatic system replacing the coil spring. The suspension fork comprises a body having a cylindrical cavity in which a piston is slidingly engaged and divides said cylindrical cavity into an upper chamber and a lower chamber. An elongated chamber extending longitudinally on the inner surface of the upper chamber, and allows gas transfer between the upper and lower chambers. The elongated chamber is arranged at the lower end of the upper chamber so that gas transfer is achieved when the suspension fork in a maximum extended position. The gas transfer from one room to the other gas enables a pressure balance of gases between the two chambers.
During the passage of the piston opposite to the oblong chamber, the latter is open simultaneously on the lower chamber and on the upper chamber, causing a significant change in stiffness during operation of the suspension system. This variation in stiffness of the suspension system can be detrimental to a user in so far as it may affect the dynamic behavior of the vehicle, causing not only inconvenience in driving comfort, but also accidents.